The present invention relates to a hydraulic pressure control apparatus for a continuously variable transmission, and particularly to an apparatus for controlling a line pressure supplied to a hydraulic cylinder of an input disc of a continuously variable transmission.
A continuously variable transmission has been proposed in which speed ratios between an input disc and an output disc, are controlled to obtain minimum fuel consumption while achieving a requisite horsepower. Such a continuously variable transmission comprises a pair of input discs, a pair of output discs, and a belt member spanning the input and output discs. A line pressure, which is supplied to a hydraulic cylinder of the output disc, is controlled according to the amount of the torque transferred between the input and output discs. Speed ratios of the continuously variable transmission are controlled by a hydraulic pressure of a cylinder of the input disc. A line pressure, which is controlled by a regulator valve, is supplied to a hydraulic cylinder of the output disc. If the value of the line pressure is too small, compared with the appropriate values necessary to transfer the torque between the input and output discs, the belt spanning the input and output discs begins slipping on the discs, thereby resulting in an inability to transfer the torque between the belt and discs. Conversely, if the value of the line pressure is too large, compared with the appropriate values, the endurance of the continuously variable transmission is shortened, and power consumed by an oil pump is lost. It is theoretically possible to obtain an optimum line pressure control if the value of the coefficient of friction on the contact surface of the belt with the discs is clarified, but the coefficient of friction varies according to oil temperature, worn belt conditions, and rotating speeds of the discs.